At a magnetic disk device, burst data indicative of position information on a track is retrieved together with sector and cylinder numbers in servo data, and a magnetic head is positioned based on the information. Typical burst patterns include a null-type burst pattern and a phase pattern. The null-type burst pattern can be defined as “ns=2, Δφ=180°”. The phase pattern can be generally defined as “ns=arbitrary number, Δφ<180°” and can be practically defined as “ns=1 or 2, Δφ≦90°, and a value which is a result of having divided 360° by a integer, where ns denotes an interval of magnetization change counted in STW step and Δφ denotes an adjacent phase difference. The phase pattern is demodulated mainly using an a tan(arctangent) demodulation technique. The null-type burst pattern is not demodulated using an a tan demodulation technique because of insufficient linearity, and thus is demodulated using a polynomial demodulation technique or the like so as to improve linearity.